1. Field of the Invention
This invention relates generally to analog signal differentiators for data detection in analog signals and, more specifically, to a sampled-time circuit for providing a precise 90-degree analog phase shift at all broadband analog signal frequencies suitable for process-independent monolithic implementation without external components.
2. Description of the Related Art
In the typical mass data storage device, digital data is stored in coded form as a sequence of signal transitions. Recovery of digital data from such storage requires read channel circuitry including means for accurate analog data signal peak detection. The analog data signal differentiator is a well-known element of analog data signal peak detectors. The precision analog differentiator provides the derivative of an analog data sense signal, which can also be understood as the 90-degree phase-shifted version of the analog data signal. That is, the derivative of an analog signal is merely an aggregate of the 90.degree. shifted sinusoidal components at all frequencies over an operating band.
The art is replete with analog signal peak detectors and signal differentiator circuits but an accurate analog signal differentiator suitable for monolithic implementation without external components is thus far unknown in the art. For instance, FIG. 1 provides a functional block diagram of a 90-degree phase shifter (differentiator) 8 employing a pair of matched filters 10 and 12. The analog signal differentiator 8 shown in FIG. 1 requires components external to the integrated circuits for implementing the matched filters.
In FIG. 1, the reference phase output signal 14 is retarded 90 degrees in phase from the quadrature output signal 16 at all frequencies. Matched filters 10 and 12 have the following characteristics (in continuous-frequency form): ##EQU1##
Thus, the resulting phase difference between the two output signals 14 and 16 is always 90 degrees at any frequency in the operating band. However, constructing filters 10 and 12 using only internal monolithic integrated circuit (IC) components introduces unacceptable variations in frequency response arising from normal variation in integrated circuit processing, thereby destroying the close matching required between them. Precision components external to the IC chip are used in the art to solve this problem. Also, because the discrete-time filter components 10 & 12 follow instead of precede the reconstruction filter, both signals 14 & 16 contain unwanted high-frequency components generated by the digital operations in filters 10 & 12, necessitating even more filters later in the system (not shown).
In U.S. Pat. No. 5,051,702, Kiyoshi Iwasaki discloses an automatic phase controlling circuit that employs a narrow-band 90-degree phase shifter for biasing the feedback loop used to control phase. Iwasaki neither considers nor suggests a method for providing accurate 90 degree phase shifts over a broad frequency band.
In U.S. Pat. No. 2,905,837, G. H. Barry discusses an early phase modulation discriminator design that relies on fixed phase shifts between subsequent signal pulses to encode information. However, Barry limits his disclosure to narrow-band operation and neither considers nor suggests precise phase control over a wide frequency band.
In U.S. Pat. No. 4,968,908, Fred L. Walls discloses a method and apparatus for wideband phase modulation and discloses a phase modulator that couples a small portion of an input signal to an amplitude modulator and phase shifter to shift the small signal portion by exactly 90 degrees. However, Walls is concerned only with a frequency and amplitude measurement system calibration technique and his operation of wideband apparatus requires that the 90-degree phase shift be manually or automatically adjusted for any particular input carrier frequency to eliminate all mixer output amplitude variations.
Another technique known in the art for wideband signal differentiation is the active analog differentiator. Although the active differentiator does not require components external to the IC chip, it suffers from increased high-frequency noise (reduced signal-to-noise ratio) because of amplitude quantization noise introduced by the digital discrete-time differentiator components. Thus, to achieve low-noise performance, an additional filter block must be added to such active differentiator, thereby introducing unwanted external components and new equalization calibration problems.
Very recently, Laber et al. ("A 20-MHz Sixth-Order BiCMOS Parasitic-Insensitive Continuous-Time Filter and Second-Order Equalizer Optimized for Disk-Drive Read Channels", IEEE Journal of Solid-State Circuits, Vol. 28, pp. 462-470, Apr. 1993) disclose a new technique for achieving a 90-degree phase difference over a range of frequencies. Laber et al. use an integrator as the final stage of an equalizer filter and, because the integrator causes a 90-degree lag at the output relative to the input, the integrator input signal may be used as a "differentiated" version of the integrator output signal. However, the circuit disclosed by Laber et al. requires an external resistor for establishing the equalizer filter operating frequency and the bandpass output emphasizes noise from all sources at the high frequency end of the operating passband.
Accordingly, there is a clearly felt need in the art for an analog signal differentiator circuit that is suitable for monolithic IC implementation over a broad frequency band without external components. The related unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.